Repeated exposure of a circuit breaker operating spring to the temperatures attained during contact arcing can cause thermal degradation of the spring which changes the operating characteristics. U.S. Pat. No. 4,011,420, entitled, "Molded Case Circuit Breaker With Improved Interrupting Capacity" describes a molded case circuit breaker which includes a barrier between the arcing chamber and the operating spring along with an arc runner to assist in controlling the extinction of the arc developed incident to an interruption. This patent is incorporated herein for purposes of reference. It has been discovered that under repeated arcing conditions, some of the gases generated during contact arcing seep through the clearance provided in the barrier for contact arm motion, and carry some of the heat generated by the arc up to the operating spring. Under extreme arcing conditions, the operating spring characteristics may become altered thereby effecting the breaker tripping mechanism.
A further prior art attempt to reduce thermal degradation of the main operating spring consisted of a moveable shutter attached to the contact arm and extending over the arcing chamber. Movement of the contact arm causes the shutter to follow and provide a baffle between the arcing chamber and the operating mechanism during contact separation.
The purpose of this invention is to provide means for protecting the circuit breaker operating spring from the high temperature gases evolved during contact arcing.